P4ROOT
P4ROOT is a fundamental environment variable used by Perforce Helix Core, a version control system. It specifies the root directory of a Perforce client workspace. When you interact with Perforce through a client, such as P4V or the command-line client, it needs to know where your local copy of the files is located. P4ROOT points to this directory.
When the Perforce client is started, it consults the P4ROOT variable to locate the workspace configuration
Setting P4ROOT is usually done when setting up a new Perforce workspace or when managing multiple workspaces